The present invention relates to thermal color printers, and more particularly to a printing head drive thereof.
In a conventional thermal color printer, four separate driving motors are used for driving an ink sheet feed mechanism, a printing paper feed mechanism and a printing head drive mechanism for lifting and lowering the printing head relative to a platen facing the printing head. The color printer of this type basically has a printing paper feed mechanism which is comprised of a paper feed roller, a printing head and a platen facing one another, and a capstan; and an ink sheet feed mechanism which feeds an ink sheet sequentially coated in the printing direction with the primary colors.
In the first forward pass, a printing paper superposed on an ink sheet passes between the platen and the printing head under pressure to thermally print a first color ink thereon. Then the printing paper printed in the first color ink is fed backward for the second forward pass for printing the second color ink thereon. When the third forward pass for printing the third color ink is finished, one printing operation of a full color print is completed. During each backward feeding operation, the ink sheet is wound for the print of the subsequent color, and the print head is temporarily separated from the platen.
Two out of the four motors are used for the printing paper feed mechanism, and the other two separate driving motors are used for the ink sheet feed mechanism and the printing head drive mechanism. Four separate motors take up a large space in a limited installation space within a thermal color printer. Consequently, this imposes a limitation on the miniaturization of thermal color printers. In order to eliminate this limitation and lower the cost of a color thermal printer of this type, an improved thermal color printer was proposed in which only one reversible motor is used for both driving the paper feed mechanism and the printing head drive mechanism. In this improved thermal color printer, one rotational direction of the reversible motor is applied to the up/down movement of the printer head relative to the platen, and the reverse rotational direction thereof is applied to driving of the ink sheet feed mechanism, and viceversa. When the motor is driven in one direction to transmit power to either the ink sheet feed mechanism or the printing head drive mechanism, power transmission to the other is cut by a one-way clutch or a one-way power transfer mechanism installed between the motor and the respective ink sheet feed mechanism and printing head drive mechanism.
Precise control of the printing head drive mechanism is required in order to stop and start the operation of lifting and lowering the printing head relative to the platen. In particular, the printing head drive mechanism has to be stopped precisely in order to provide a proper contact of the printing head with the top surface of the platen for an optimum printing. However, this one-way clutch or one-way power transfer mechanism has a substantially large inertia. This inertia is very difficult to control, and without control of the inertia, the printing head drive mechanism overruns due to the inertia of the one-way clutch. Further, conditions of controlling the printing head drive mechanism, such as switching on and off of the motor, had to be decided based on experiments, which likely renders the control unreliable.